1) Download the firmware from Adtran’s site and place it in the root directory of the tftp sever path
2) Plug the serial port into the Adtran and your computer
3) Plug the patch cable into port ETH 0/1 on the Atran and then into your computer
4) Assign a private static IP to your computer (in this example use 10.10.10.5)
5) Start the terminal client and set it to 9600, 8, 1, None, None (make sure the com port matches your serial port)
6) Plug in the Adtran
7) When prompted (about 5 seconds into the boot sequence) hit Esc to enter the bootstrap; you will now be at the bootstrap# prompt.
8) Type “erase file-system”
9) Type “ip address 10.10.10.1 255.255.255.0”
10) Type “copy tftp flash” and you will get the following prompts:
Address of remote host? 10.10.10.5
Source filename? {the name of the firmware file}
Destination filename? {hit enter}
It will then transfer the file from your PC to the Adtran
11) Rename the default config file to startup-config and repeat step 10 (except the filename is now startup-config) to upload the config file to the Adtran
12) Type “boot system flash {the name of the firmware file} verify”
13) Type “boot”

The system will now boot the new firmware with the default config.
I highly recommend watching the entire boot process and see if there are any error messages.